Inverters and converters are known as power converters. Inverters convert direct current into alternating current and supply the alternating current to a load such as a motor. Converters convert alternating current supplied from an alternating current power source into direct current.
As an approach for obtaining a sine wave output voltage from a three-phase inverter, Pulse Width Modulation (PWM) control is generally used, for example. In the PWM control, switching operations are performed at a high speed by high-side switches and low-side switches connected in parallel in each of U-phase, V-phase, and W-phase arms. This contributes to generation of high-frequency switching noise of the three-phase inverter. Further, a load such as a motor has parasitic capacitance between the motor itself and a frame ground, and accordingly the high-frequency switching noise might flow through the motor via the parasitic capacitance and cause damage to a bearing of the motor and malfunction of the accessories. So as to solve such a problem, various technologies for reducing noise have been disclosed (e.g., Patent Literature 1).
Meanwhile, in recent years, power converters represented by inverters and converters have been required to be large in capacity. However, the ampacity of a switching device used therein is limited. In consideration of this issue, as shown in FIG. 23, there has been a technology for using a plurality of switching devices connected in parallel.
FIG. 23 shows an overall structure of a load drive system 900 provided with a power converter pertaining to conventional art.
The load drive system 900 converts direct current supplied from a direct current power source DC into the three-phase alternating current by using a three-phase inverter 902, and supplies the three-phase alternating current to a motor 904. The operations of the three-phase inverter 902 are controlled by PWM signals Pu, Pv, and Pw that are output from a controller 905.
The three-phase inverter 902 constitutes a three-phase bridge, and is composed of a U-phase arm 912u, a V-phase arm 912v and a W-phase arm 912w. The U-phase arm 912u is composed of a high-side switching device group Q91 and a low-side switching device group Q92 connected in parallel. The high-side switching device group Q91 is composed of switching devices Q91a and Q91b connected in parallel. In a similar manner, a low-side switching device group Q92 is composed of switching devices Q92a and Q92b connected in parallel.
A gate terminal of each of the switching devices Q91a, Q91b, Q92a and Q92b is connected to a gate drive circuit GD. The switching devices connected in parallel such as the switching devices Q91a and Q91b are supplied with the same PWM signal Pu from the controller 905 via their respective corresponding gate drive circuits GDs. As a result, the switching devices Q91a and Q91b operate synchronously with each other. Note that although reference signs are not particularly provided in FIG. 23, the V-phase arm 912v and the W-phase arm 912w have the same structure as the U-phase arm 912u. 
According to the above structure, the switching device group can increase an amount of current to be flowed, compared with a case in which one switching device group is composed of only one switching device. Even when one switching device group is desired to flow a predetermined amount of current, no single switching device can send the predetermined amount of current. In such a case, it is possible to satisfy the above desire by using the above technology, that is, by forming one switching device group by connecting in parallel two switching devices each of which can flow approximately a half of the above predetermined amount of current.